Everyone in the communications technology area is aware of the increasing interest in multi-media networks and systems. As far as communications are concerned, multi-media processing means merging and distributing, without impairment, text, images, and speech over a same high speed network covering very long distances, This implies efficiently converting all three kinds of information into digitally encoded data and then vehiculating those data over a common network. Needless to mention that several important technical problems have to find solutions prior to one being able to provide acceptable networks.
Echo is one of those problems, and echo cancellation the solution. Several echo cancelers are already available. They, however, are not yet fully satisfactory from a cost/efficiency standpoint, particularly when multiple echoes are involved.
To make the above statements clearer, one should remind the basic principles. Speech signals issuing from the telephone set are first transmitted, at least over a short distance, in their analog form and through a bi-directional two-wire line. Said line is then split into two mono-directional lines (four wires overall). One mono-directional line for incoming signals, the other for outgoing signals. Analog-to-Digital (A/D) conversion of the signal are operated over the outgoing speech signal, to enable further transmission over a high speed digital network. Then correlative Digital-to-Analog (D/A) conversion need being performed at the receiving end prior to the signal being vehiculated to the destined telephone set over a bi-directional 2-wire line. Therefore, at least at both ends of the network, two-to-four and four-to-two wire conversions must be performed. These are conventionally achieved through so-called hybrid transformers. Due to unavoidable hybrid load mismatching throughout the voice frequency bandwidth parts of outgoing speech are fedback to their originating telephone user. These are the so-called echoes which do disturb significantly the communication between end-users.
Obviously, each hybrid transformer on the speech path may generate an echo, which worsens the situation. Simple echoes are already difficult to track and cancel, therefore, needless to insist on the complexity of the problem when dealing with multiple echoes and/or when dealing with variable environments due to PBX's.
Echo cancelers are made using adaptive digital filters monitoring the digital signals over both mono-directional lines, processing said signals to generate an echo replica and subtracting said replica from the actual echo polluted signal. The echo replica generating filter (so-called echo filter) coefficients are dynamically estimated and adjusted to tune the filter. The coefficients estimation computing load is fairly heavy, which is a main drawback with current signal processors.
The higher the number of required coefficients, the higher the filter complexity. In addition, one should remember that the echo filter should be designed to dynamically synthesize the echo path impulse response h(t). The filter coefficients may be defined as samples of said impulse response. The longer the impulse response, or more particularly the significant portion of said impulse response, the higher the number of coefficients to be considered.
In addition, and especially in a variable network environment, the coefficients computation should be fast enough to render the system operable satisfactorily. This could be achieved, at least partially, by increasing the signal processor power. But in practice, to optimize efficiency versus complexity, one needs optimizing the coefficients computation.
Otherwise, the computing workload involved might render the whole echo canceler cost prohibitive.
Different methods have been proposed for tracking the significant portions of echo path impulse response, or for computing echo filter coefficients. They all either lack efficiency on adaptiveness or lack efficiency in their convergence toward the right coefficients value.
One object of this invention is to provide a fully adaptive method to track and compute the most significant echo filter coefficients directly through dynamic threshold considerations, to adapt to changing network environment as required.
These and other objects, characteristics and advantages of the present invention will be explained in more detail in the following, with reference to the enclosed figures, which represent a preferred embodiment thereof.